1. Field of the Invention
The present invention relates to a method for testing of the insulating ability and of an insulation on an electric conductor, such as an enameled wire or a cable, and of the type.
2. Description of the Prior Art
U.S. Pat. No. 3,413,541 discloses an apparatus, where the difference of voltage for the testing is established between the electric conductor and a mechanical arrangement with rolls; and from U.S. Pat. No. 2,488,578 it is furthermore known to use hanging chains to form connection to the conductor, which insulation has to be tested. Furthermore it is known to transfer the testing voltage to the conductor by means of metal balls or the like, through which the conductor is moved.
These methods have in common that an insulation defect on the conductor, for instance in the form of a crack or a hole, will be registered as a voltage flashover. The voltage has to be chosen so that it does not exceed the puncture voltage of the insulating material, but is larger than the puncture voltage in atmospheric air at the same distance as the thickness of the insulating material.
It is furthermore known to use a contact wheel, where the wire to be insulation tested extends in an angular notch along the periphery of the wheel. That is that puncture may only occur where the sides of the notch touch the wire. An arrangement, where the wheel is substituted by a conducting media, such as salt water or mercury, which is known from the description in Danish patent application No. 2281/88, does not have this defect; but causes pollution of the wire to an unacceptable degree, which requires a subsequent cleaning process. The liquid may be substituted by a metal powder or by metal balls. This will not cause pollution of the surface of the wire; but the surface of the wire may be damaged by grinding.
WO 90/05311 furthermore discloses a test method making use of a superimposed high A.C. voltage on the testing voltage, whereby a puncture may be obtained where the insulation layer is defective. This requires that the superimposed voltage is adjusted to a voltage between the puncture strength for air, or in this case a gas which ionizes easily, Argon, and for the measured insulating material. However, in this case ionizing only occurs if a defect in the insulation occurs.
DD-A-209 912 and DD-A-209 913 discloses a test method, where the conducting media also may be replaced by a sufficient quantity of electric charge carriers, ions, which over a suitable axial distance is arranged to surround the wire. Such an electric conducting cloud may be a plasma cloud, or ionized gaz. A plasma cloud generated by a lighted electric arc is capable of carrying an electric current, by ionizing of for instance a noble gas between the electrodes. An electric arc is aggressive, both towards the electrode materials, the insulating materials and in this connection the item tested, due to the very high temperature to maintain the arc, and thereby the ionizing. This results in unfavourable properties in he form of sporadic damage of the test item and the insulating material and poor durability of the electrodes. However the test method disclosed in DD-A-209 912 is simplified by obtaining a varying division of the supplied voltage and with it a potential displacement between the light arc and the test voltage by means of an ohmic voltage divider.
U.S. Pat. No. 4,151,461 discloses, that the ionizing also may be obtained by continuously discharging of sparks; but to secure continuous ionizing it requires high power, 150-800 watts at 100-600 Hz, and it therefore causes that the electrodes to be quickly worn down.
U.S. Pat. No. 3,263,165 furthermore discloses, that the ionizing of atmospheric air also may be provided by means of a radioactive isotope, which may establish a current through the atmospheric air. Due to environmental reasons is not expedient, and requires intensive maintenance.
EP-A-0 264 482 discloses the use of laserbeams to generate the charge carriers, and EP-A-0 367 379 furthermore discloses, that electron emitting materials, such as chemical compounds of barium or calcium may emit electrons--but only by heating to high temperatures.
The aforementioned six methods have the important advantage, that the insulating material and the wire are not touched mechanical, which is why there is neither wear, tearing or physical damage to the surface of the insulation. Hardening of the metal core of the item, which normally may happen by mechanical bending around rolls or the like, does not occur. Such hardening is very unfavorable such as, for example, copper wire for spools in electric machines etc.